The invention relates to a device for measuring the wall thickness of an extruded tubular section during extrusion, comprising a tube wall thickness measuring instrument being rotatable about the periphery of the tube, and regulating means for causing the measuring instrument to be pressed against the wall of the tubular section.
Such a device for measuring the wall thickness of an extruded tubular section is known and presents the advantage that during the extrusion the wall thickness of the extruded tubular section can be measured continuously and on the basis of such measurements measures can be taken to prevent an extruded tubular section with an incorrect wall thickness from being obtained, which leads to undesired material consumption.
In this known device for measuring the wall thickness of an extruded tubular section the measuring instrument is fitted hingedly on the end of an arm. This arm can itself pivot in a plane parallel to and through the axis of the tube, and said hinge is fastened in a guide ring extending perpendicular to the axis of the tube and concentrically about a tubular section to be extruded, so that the hinge can turn about the tubular section. By means of a spring, the measuring instrument and a supporting point situated upstream of the measuring instrument are pressed against the wall of the tubular section.
This known device presents the disadvantage that gravity has a great influence on the measurement results. For, in practically all extrusion apparatus the axes of the tubes are positioned horizontally, which means that at one moment the measuring instrument is lying below the extruded tubular section, in which case the pressure force is opposed to the force of gravity, while some time later the measuring instrument is above the extruded tubular section, where the pressure force and the force of gravity are in fact working in the same direction.
This means that the difference in application force of the measuring instrument is twice the weight of the measuring instrument itself and half the arm, so that the maximum pressure force with which the measuring instrument has to be pressed against the wall of the tubular section must be more than said difference. In many cases this has proved to be too much, so that the measuring instrument is damaged or the extruded tubular section is deformed and in any case the measurements are not accurate enough, while in certain cases breakdowns may even occur.
Another problem is that in this known device the measuring instrument is supported on the wall of the tube by four supporting elements fixed relative to one another, so that angular rotation, out-of-true settings and the like which are generally due to wear and/or jamming of points of rotation or hinge points mean that all the supporting elements do not always rest on the wall of the tubular section, which gives rise to faulty measurements.